I will start this post with a foreword that at least one of the protocols in the title, CSMA/CD, is obsolete. I’m including it here as it’s very useful to understand why we no longer need it, due to the changes in layer 2 topologies that have evolved over time. A little history can illuminate why we are where we are.

CSMA (Carrier Sense Multiple Access) is a methodology that deals with multiple computing nodes access the same physical media, whether that be a piece of wire, optical fibre or even the air. It makes sense that there should be some rules around when each node can transmit/receive rather than a free-for-all where interference, possible corruption and inefficiency can occur. The media is being shared so access needs to be given through arbitration.